The present invention relates generally to the display of information on raster scanned cathode ray tubes. A preferred embodiment of the invention is incorporated in a scanning electron microscope. While the features of applicant's invention are subject to a wide range of application, they will be specifically illustrated herein in the context of their application within the display system of a scanning electron microscope, and in particular, a field emission scanning electron microscope.
One of the very significant advantages of scanning electron microscopy systems is the inherent ability to directly view the investigated portion of a specimen on a cathode ray tube type monitor. This real time viewing of the specimen permits the practitioner to gain much valuable information. It may be appreciated that there will be many instances when it is not only desirable to view a selected specimen in real time, but also concurrently preserve the viewed image for investigation in greater detail at susequent times. In this latter regard, conventional scanning electron microscope systems offer camera attachments to the cathode ray tube monitors so that photographic records of the viewed images may be obtained.
It should be appreciated that in the preparation of a photographic record of an image being viewed that the highest degree of resolution be maintained. This is advantageous since the photographs are often further enlarged to take advantage of the high resolution of the photographic film. It should be further appreciated that in order to accomplish high resolution in the ultimate photograph, the visual presentation on the cathode ray tube from which the photographic record is prepared should be as stable and as free of "noise" as possible. This stability includes a satisfactory signal to noise ratio for intelligible transmission of information, a stability of picture information on the screen directly coordinated and synchronized with the scan of the electron beam of the scanning electron microscope across the surface of the specimen, and a stability in the contrast and brightness of the corresponding similar gray levels of different spots on the specimen's surface when interrogated by the electron beam. It is particularly with the latter in mind that the present invention was conceived.
Briefly, as illustrated in the figures and subsequently described in the specification, applicant's invention contemplates improvements to a field emission scanning electron microscope display system. A field emission gun generates a beam of charged particles which are accelerated and focused to impinge upon a specimen undergoing investigation. A detector suitably located with respect to the surface of the specimen produces an image signal relative to the impingement of the beam upon the specimen and the scattering of reflected electrons, secondary electrons and X-rays from the surface of the specimen. A viewing tube operatively associated with the detector displays an image of the specimen upon the sensitized face of the tube. This image is produced by the modulation of the viewing tube electron beam in accordance with the varying amplitude of the image signal being provided by the detector. A control synchronously deflects the charged particle beam of the electron microscope with the viewing tube beam in a predetermined scanning pattern. In the case of the systems where a photograph record is desired and wherein simultaneous real time viewing is preferred, the display system may include two cathode ray tube-type viewing tubes, the electron beams of which are both synchronously deflected in accordance with the charged particle beam of the electron microscope.
Among the features of the present invention is the electronic circuitry which is operatively associated with the viewing tube from which a photographic record of the image being investigated is produced. It is the function of this electronic circuitry to insure that the blackest or darkest portions of the specimen investigated by the charged particle beam of the electron microscope are correspondingly presented as the blackest or darkest portions on the viewing tube and correspondingly that the whitest portions of the investigated specimens are recorded on the viewing tubes as the whitest portions of the presented image.
It is also an object of the invention that the "blackest black" which is observed on the specimen's surface is continually remembered and observed on the viewing tube as the blackest black, and that all shades or gray levels above the black are simultaneously and immediately represented by a signal value corresponding to their relative relationship to the blackest black and further that the range from black to white as presented on the surface of the specimen is continually ranged during the period of investigation over the black to white range presented on the viewing tube.